In conventional art a liquid crystal display device provides the display device used for a mobile telephone or the like. In this kind of liquid crystal display device a backlight device providing a light source together with a light guide plate that guides light emitted from this light source is used in order to provide illumination from the rear face of liquid crystal display elements.
FIG. 1 provides an external view of a conventional light guide plate. FIG. 1 (a) provides a view of the light guide plate from above, FIG. 1 (b) provides a side view of the light guide plate and FIG. 1 (c) provides a perspective view of the light guide plate. In those drawings the light source is provided by light emitting diode 102.
The light guide plate 101 is comprised of a transparent material such as PMMA or polycarbonate or the like and has a substantially flat, planar form. The upper face of the light guide plate 101 comprises the exit face 103, the lower face comprises the reflective face 104 and one of the side faces comprises the entry face 105. Reflective elements 106 that operate to reflect light incident to the entry face 105 toward the exit face 103 are formed on the reflective face 104.
Light output from the light emitting diode 102 light source enters the light guide plate 101 from the entry face 105 and proceeds within the light guide plate 101 continually undergoing total reflection at the exit face 103 and the reflective face 104 until the angle of the exit face 103 and the reflective face 104 reaches a critical angle. The reflective elements 106 formed on the reflective face 104 perform the role of deflecting light in the direction of the exit face 103. Light traveling in the light guide plate 101 is deflected in the direction of the exit face 103 as it is reflected at the reflective elements 106 and exits from the exit face 103 if the angle formed between traveling direction of light and the exit face 103 exceeds a critical angle. The light guide plate 101 in which light input from the entry face 105 comprising a side face is output from the exit face 103 comprising the main face is called a side edge type, and is widely used for mobile telephone devices and the like.
FIG. 2 is a cross section showing the condition of usage of a conventional light guide plate and backlight device.
The light guide plate 101 is arranged such that directly below the liquid crystal display elements 107, the exit face 103 is disposed opposing the lower face 109 of the liquid crystal display elements 107 with the optical sheet 108 positioned between the exit face 103 and that lower face. In the light guide plate 101, light from the light emitting diode 102 enters from the entry face 105.
Light from the entry face 105 incident to the light guide plate 101 travels inside the light guide plate 101 while undergoing total reflection at the exit face 103 and the reflective face 104 until the angle between traveling direction of light and the exit face 103 reaches a critical angle. The reflective elements 106 formed on the reflective face 104 fulfill the role of deflecting the light in the direction of the exit face 103. The reflective elements 106 formed on the reflective face 104 perform the role of deflecting light in the direction of the exit face 103. Light traveling in the light guide plate 101 is deflected in the direction of the exit face 103 as it is reflected at the reflective elements 106 and exits from the exit face 103 if the angle formed between traveling direction of light and the exit face 103 exceeds a critical angle.
Light output from the exit face 103 of the light guide plate 101 enters the lower face 109 of the liquid crystal display elements 107 via the optical sheet 108. The optical sheet 108 directs the light output from the light guide plate 101 up in the direction of the liquid crystal display elements 107 such that the light is incident vertically to the lower face 109 of the liquid crystal display elements 107.
Further, part of the light that enters the light guide plate 101 is output from the reflective face 104 when the angle formed between itself and the reflective face 104 exceeds a critical angle. In order to reuse this leaked light a reflector is disposed positioned opposing the reflective face of the light guide plate. The light leaked from the light guide plate is reflected at this reflector and enters from the reflection face of the light guide plate once again before being output from the exit face of the light guide plate.
Patent documents disclosing this conventional art include for example Japanese Unexamined Patent Application Publication No. 2000-222924 and Japanese Unexamined Patent Application Publication No. 2000-211426.